Converter and control method thereof

ABSTRACT

A converter includes a transformer, an active clamp circuit, a primary side switch, a secondary side switch, a load detection circuit, a state detection circuit and a control circuit. The active clamp circuit is electrically coupled to the primary winding of the transformer, and includes a clamp switch. The primary side switch is electrically coupled to the primary winding and a primary ground terminal. The secondary side switch is electrically coupled to a secondary winding of the transformer and a load. The control circuit outputs a control signal to turn on or turn off the clamp switch. The control circuit sets a blanking time according to the load state signal, such that the clamp switch is turned on when a drain-source voltage of the primary side switch is at a peak value of a resonance after the blanking time starting from the reference time point.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Continuation-in-part of the applicationSer. No. 16/835,331, filed on Mar. 31, 2020, which is a divisional ofthe application Ser. No. 16/246,539, filed on Jan. 13, 2019, U.S. Pat.No. 10,644,606, and claims priority under 35 U.S.C. 119(e) to Chinaapplication serial number 201810194211.1, filed on Mar. 9, 2018, theentire contents of which are hereby incorporated herein by reference intheir entireties.

BACKGROUND Technical Field

The present disclosure relates to a converter, especially with regard toa flyback converter.

Description of Related Art

In recent years, switching power supply has been widely applied toportable mobile devices such as laptops, tablet computers, smart phones,and so on. The miniaturization, high efficiency, and high frequency arethe trend of switching power supply.

Wherein the flyback converter with QR control mode has been widely usedin the low power field, especially applied for the application withpower less than 100 W, because of simple circuit structure, low cost,and low switching loss with valley turning on.

However, the conventional QR control mode of flyback converter is notsuitable for the development trend of miniaturization and high switchingfrequency due to the switching loss increased rapidly with highswitching frequency. In order to decrease the switching loss, a newcontrol method of flyback converter is provided.

SUMMARY

One aspect of the present disclosure is provided a converter includes atransformer, an active clamp circuit, a primary side switch, a secondaryside switch, a load detection circuit, a state detection circuit and acontrol circuit. The transformer includes a primary winding and asecondary winding. The active clamp circuit is electrically coupled tothe primary winding, and comprising a clamp switch. The primary sideswitch is electrically coupled to the primary winding and a primaryground terminal. The secondary side switch is electrically coupled tothe secondary winding and a load. The load detection circuit isconfigured to detect a load state and correspondingly output a loadstate signal. The state detection circuit is configured to detect areference time point. The control circuit is configured to output acontrol signal to turn on or turn off the clamp switch. The controlcircuit is configured to set a blanking time according to the load statesignal, such that the clamp switch is turned on when a drain-sourcevoltage of the primary side switch is at a peak value of a resonanceafter the blanking time starting from the reference time point.

Another aspect of the present disclosure is provided a control method ofa converter, wherein the converter comprises a primary side switch, atransformer, a secondary side switch and an active clamp circuit, thetransformer comprises a primary winding and a secondary winding, theactive clamp circuit is electrically coupled to the primary winding, andthe active clamp circuit comprises a clamp switch. The control methodincludes: detecting a load state by a load detection circuit andcorrespondingly outputting a load state signal; setting a blanking timeby a control circuit according to the load state signal; detecting areference time point by a state detection circuit; and outputting acontrol signal to a clamp switch of the converter by the control circuitso as to turn on the clamp switch when a drain-source voltage of theprimary side switch is at a peak value of a resonance after the blankingtime starting from the reference time point.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the disclosure as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure can be more fully understood by reading thefollowing detailed description of the embodiment, with reference made tothe accompanying drawings as follows:

FIG. 1 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 2 is waveforms of the first control signal Sc1, the second controlsignal Sc2, the primary side current Ip, the secondary side current Is,the drain-source voltage Vds1 of the primary side switch, and thedrain-source voltage Vds2 of the secondary side switch of the converterin some embodiments of the present disclosure.

FIG. 3 is a schematic diagram of the relation chart between the loadstate signal and the length of the blanking time in some embodiments ofthe present disclosure.

FIG. 4A is a schematic diagram of the waveforms of the heavy load insome embodiments of the present disclosure.

FIG. 4B is a schematic diagram of the waveforms of the medium load insome embodiments of the present disclosure.

FIG. 4C is a schematic diagram of the waveforms of the light load insome embodiments of the present disclosure.

FIG. 5 is a schematic diagram of voltage and current waveforms of theconverter in some embodiments of the present disclosure.

FIG. 6 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 7 is waveforms of the secondary side current Is, the drain-sourcevoltage Vds1 of the primary side switch, the cross voltage Vaux of theprimary auxiliary winding, trigger signal Sa and start signal TB_startin some embodiments of the present disclosure.

FIG. 8 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 9 is waveforms of the first control signal Sc1, the drain-sourcevoltage Vds1 of the primary side switch, the secondary side current Is,the second control signal Sc2, the drain-source voltage Vds2 of thesecondary side switch and turn on signal Vy in some embodiments of thepresent disclosure.

FIG. 10 is a flowchart illustrating a control method in some embodimentsof the present disclosure.

FIG. 11 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 12 is waveforms of the first control signal S1, the third controlsignal Sc3, the primary side current Ip, the secondary side current Is,and the drain-source voltage Vds1 of the primary side switch of theconverter in some embodiments of the present disclosure.

FIG. 13 is a schematic diagram of the waveforms of the medium load insome embodiments of the present disclosure.

FIG. 14 is a schematic diagram of the waveforms of the light load insome embodiments of the present disclosure.

FIG. 15 is a schematic diagram of voltage and current waveforms of theconverter in some embodiments of the present disclosure.

FIG. 16 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 17 is waveforms of the secondary side current Is, the drain-sourcevoltage Vds1 of the primary side switch, the cross voltage Vaux of theprimary auxiliary winding, trigger signal Sa and start signal TB_startin some embodiments of the present disclosure.

FIG. 18 is a schematic diagram of a converter in some embodiments of thepresent disclosure.

FIG. 19 is a flowchart illustrating a control method in some embodimentsof the present disclosure.

DETAILED DESCRIPTION

For the embodiments below is described in detail with the accompanyingdrawings, embodiments are not provided to limit the scope of the presentdisclosure. Moreover, the operation of the described structure is notfor limiting the order of implementation. Any device with equivalentfunctions that is produced from a structure formed by a recombination ofelements is all covered by the scope of the present disclosure. Drawingsare for the purpose of illustration only, and not plotted in accordancewith the original size.

It will be understood that when an element is referred to as being“connected to” or “coupled to”, it can be termed “electrically connectedto” or “electrically coupled to”, and it can be directly connected orcoupled to the other element or intervening elements. As used herein,the term “and/or” includes an associated listed items or any and allcombinations of more. In addition, although terms such as “first”,“second” are used to describe different elements, it should beunderstood that such words are used to distinguish elements oroperations which are described using the same terminology. Unlessotherwise stated, such words are not intended to imply any specificorder or sequence or to limit the scope of the present disclosure.

Referring to FIG. 1, the converter 100 is configured to convert an inputvoltage Vin received from an input voltage source into an output voltageVo. In some embodiments, the converter 100 may be a flyback converter,but the present disclosure is not limited thereto.

As shown in FIG. 1, the converter 100 includes transformer 110, aprimary side switch S1, a load detection circuit 120, a state detectioncircuit 130 and a control circuit 140. The transformer 110 includes aprimary winding M1 and a secondary winding M2. The transformer 110 isconfigured to transmit the received power from the primary winding M1 tothe secondary winding M2. Structurally, the first terminal of theprimary winding M1 is electrically coupled to the positive terminal ofthe input voltage Vin. The second terminal of the primary winding M1 ofthe transformer 110 is electrically coupled to the first terminal of theprimary side switch S1. The second terminal of the primary side switchS1 is electrically coupled to a primary ground terminal (or negativeterminal of input voltage Vin). In other words, the primary side switchS1 is electrically coupled between the primary winding M1 and theprimary ground terminal. The control terminal of the primary side switchS1 is configured to receive a first control signal Sc1 to turn on orturn off the primary side switch S1. For example, the primary sideswitch S1 turns on when the first control signal Sc1 has a first level(e.g., high level). Relatively, the primary side switch S1 turns offwhen the first control signal Sc1 has a second level (e.g., low level).

Further, the converter 100 includes a secondary side rectifier circuit.As shown in FIG. 1, the secondary side rectifier circuit includes asecondary side switch S2, and the secondary side switch S2 is connectedbetween the secondary winding M2 and load. Further, the secondary sideswitch S2 is electrically coupled to the first terminal of the secondarywinding M2 and the first terminal of the output capacitor Co. In someother embodiments, the secondary side switch S2 can be arranged betweenthe second terminal of the secondary winding M2 and the second terminalof the output capacitor Co. The control terminal of the secondary sideswitch S2 receives a second control signal Sc2 to control the secondaryside switch S2 on or off. Wherein, the secondary side switch S2 may beMOSFET, IGBT or GaN devices. In some other embodiments, the secondaryside switch S2 may be a diode or other components.

Specifically, when the primary side switch S1 is turned on, a primaryside current Ip flows through the primary winding M1 of the transformer110, and correspondingly stores the energy in the transformer 110. Atthis time, the polarity of the secondary winding M2 of the transformer110 is opposite to the polarity of the primary winding M1, and thesecondary side switch S2 is off. No current flows through the secondaryside switch S2, and no energy is transferred from the primary winding M1to the secondary winding M2. The energy received by the load is providedby the output capacitor Co.

Relatively, when the primary side switch S1 is turned off, the polarityof the windings will reverse. At this time, the secondary side switch S2conducts so as to the energy of the transformer 110 transfers to thesecondary winding M2 from the primary winding M1 and forms a secondaryside current Is. The secondary side current Is flows through thesecondary side switch S2, such that the energy stored in the transformer110 transmits to the load and output capacitor Co through the secondaryside switch S2.

When the energy of the transformer 110 is transferred to the load andthe output capacitor Co, the secondary side current Is is graduallydecreased. The primary winding M1 includes a magnetizing inductance Lmand a leakage inductance Lk. When the secondary side current Is drops tozero, the parasitic capacitor C1 of the primary side switch S1 willresonate with the magnetizing inductance Lm, resulting in correspondingoscillation of the drain-source voltage Vds1 of the primary side switchS1. Then, the primary side switch S1 of the converter turns on throughthe first control signal Sc1 again, so that the primary side current Ipflows through the primary winding M1 to store energy to the transformer110. Accordingly, by repeatedly controlling turn on or turn off of theprimary side switch S1 and the secondary side switch S2, the converter100 can convert the input voltage Vin into the output voltage Vo.

In order to decrease the switching loss, the optimal time to turn on theprimary side switch S1 is when the drain-source voltage Vds1 of theprimary side switch S1 at valley of the resonance. In the presentdisclosure, the state detection circuit 130 is configured to detect areference time point. The reference time point is corresponding to atime point when the secondary current Is in the secondary winding M2drops to zero. The load detection circuit 120 is configured to detectthe state of the load, and correspondingly outputs a load state signalVfb. The control circuit 140 confirms the present load state of theconverter 100 is in a light load state, a medium load state or a heavyload state according to the load state signal Vfb so as to set ablanking time. Then the control circuit 140 outputs a first controlsignal Sc1 to turn on the primary side switch S1 when the drain-sourcevoltage Vds of the primary side switch is at a valley of the resonanceafter the blanking time starting from the reference time point.

Accordingly, since the control circuit 140 sets the blanking timeaccording to the load state, and the blanking time is not affected bythe switching frequency of the primary side switch S1. The convertercontrol method of the present disclosure is compatible with theconverter 100 with any switching frequencies design.

According to one aspects of the present application, the converter 100includes a clamp circuit 150 which includes a clamp resistor R3, a clampcapacitor C3 and a diode D1. The clamp circuit 150 is parallel to theprimary winding M1 and configured to clamp the drain-source voltage Vds1of the primary side switch S1 when the primary side switch S1 is turnedoff.

Referring to FIG. 2, similar elements related to the embodiment of FIG.1 are assigned with the same reference numerals for betterunderstanding. For convenience and clarity, the waveforms of the firstcontrol signal Sc1, the second control signal Sc2, the primary sidecurrent Ip, the secondary side current Is, and the drain-source voltageVds1 of the primary side switch S1 of the converter 100 shown in FIG. 2will be described with the embodiments shown in FIG. 1, but not limitedthereto.

At the time point t0, the secondary side switch S2 is turned on. Duringthe time t0 to t1, the converter 100 is in the state of transferringenergy to the load from the transformer 110, and the secondary currentIs decreases gradually. At the time point t1, the secondary current Isdrops to zero, and the first control signal Sc1, the second controlsignal Sc2 both are keeping low level, and the secondary side switch S2is turned off. The drain-source voltage Vds1 of the primary side switchS1 starts to oscillate. Therefore, the time point t1 is the “referencetime point” described above.

During the time t0 to t1, if the load detection circuit 120 detects thatthe converter 100 is in heavy load, the control circuit 140 will set theblanking time equal to zero. When the control circuit 140 detects thatthe drain-source voltage Vds1 of the primary side switch starts tooscillate and at a valley of the resonance for the first time (timepoint t2), the control circuit 140 outputs the first control signal Sc1(e.g., becomes high level) to turn on the primary side switch S1. Duringthe time t2-t3, the first control signal Sc1 is in high level and theprimary side switch S1 is turned on so as to allow the primary currentIp flows through the primary winding M1 and primary side switch S1.Therefore, the drain-source voltage Vds1 of the primary side switch S1is zero.

At the time point t3, the first control signal Sc1 switches from highlevel to low level. Correspondingly, the primary side switch S1 isturned off and the primary current p becomes zero. During the timet3-t4, the secondary current Is flows through the secondary side switchS2. As the energy stored on the transformer 110 is transferred to theload, the secondary current Is will gradually decrease from its maximumvalue to zero.

In some embodiments, for example, if the load detection circuit 120detects that the converter 100 is in medium load. Starting from the timepoint t4, the control circuit 140 sets a blanking time. After theblanking time, when the drain-source voltage Vds1 of the primary sideswitch S1 is at a valley of the resonance again (e.g., the time point t5in FIG. 2), the control circuit 140 generates the first control signalSc1 to turn on the primary side switch S1 again. The above time pointst2-t5 may be considered as one of the working period of the converter100. By repeatedly controlling the primary side switch S1 and thesecondary side switch S2 to turn on or turn off, the converter 100 canconvert the input voltage Vin into the output voltage Vo and output itto the load.

Specifically, the converter 100 sets different length of the blankingtime according to the load state signal Vfb. Wherein, the length of theblanking time increases as the load state decreases. That is, there isnegative correlation between the blanking time and the magnitude of theload state signal. For example, the converter 100 may work in the heavyload state, the medium load state and the light load state. When theconverter 100 is in the heavy load state, the control circuit 140selects a heavy load time as the blanking time. When the converter 100is in the medium load state, the control circuit 140 select a mediumload time as blanking time. The medium load time is longer than theheavy load time. When the converter 100 is in light load state, thecontrol circuit 140 selects a light load time as the blanking time. Thelight load time is longer than the medium load time. When the converter100 is in very light load, the control circuit 140 generates a firstcontrol signal to turn on the primary side switch S1 after a longerblanking time starting from the reference time point without consideringthe valley.

Referring to FIG. 3, FIG. 3 is the relation chart between the load statesignal Vfb and the length of the blanking time in some embodiments ofthe present disclosure, wherein the horizontal axis is the load statesignal Vfb which represents the load state of the converter 100. Thevertical axis is the blanking time which represents the length ofblanking time that should be set. As shown in FIG. 3, the characteristicline of relation chart is like a ladder. There are multiplecorresponding critical values V10, V11, V21, V20-V60, V61 on thehorizontal axis and multiple corresponding blanking times TB1-TB6 on thevertical axis.

The control circuit 140 adjusts the length of blanking time along withthe trend of the ladder shaped relationship line. For example, as shownin FIG. 3, when the load state signal Vfb decrease to the critical valueV21, the control circuit 140 adjusts the blanking time from TB1 to TB2.When the load state signal Vfb increase to the critical value V20, thecontrol circuit 140 return the blanking time from TB2 to TB1.

The heavier load state (e.g., the load state signal Vfb become larger),the shorter blanking time. In other words, the lighter load state (e.g.,the load state signal Vfb becomes smaller), the longer blanking time. Inthis way, the control circuit 140 can adjust the length of the blankingtime, according to the magnitude of the load state signal Vfb.

Referring to FIG. 4A-4C, FIG. 4A-4C are waveforms of drain-sourcevoltage Vds1 of the primary side switch S1, blanking signal TBx, turn onsignal Vy, and the first control signal Sc1 of the converter 100 in the“heavy load state”, “medium load state” and “light load state”. As shownin FIG. 4A, the blanking time is zero in the heavy load state, so thatthe blanking signal TB0 in the control circuit 140 maintains to highlevel. When the control circuit 140 detects that the drain-sourcevoltage Vds1 of the primary side switch S1 is at a valley of theresonance, the control circuit 140 generates the first control signalSc1 to turn on the primary side switch S1. In some embodiments, thecontrol circuit 140 detects the drain-source voltage Vds1 of the primaryside switch S1 by a valley detection circuit 141. When detecting thatthe drain-source voltage Vds1 of the primary side switch S1 is at avalley of the resonance, the control circuit 140 generates a turn onsignal Vy to the control circuit 140.

Similarly, as shown in FIG. 4B, in the medium load state, the controlcircuit 140 sets the blanking signal TB2 to low level from the referencetime point, and during the time when the blanking signal TB2 is lowlevel, the turn on signal Vy does not work. After the blanking timestarting from the reference time point, the control circuit 140 sets theblanking signal TB2 to high level. At this time, when the valleydetection circuit 141 detects that the drain-source voltage Vds1 of theprimary side switch S1 is at a valley of the resonance, the controlcircuit 140 outputs the first control signal Sc1 (e.g., to be highlevel) to turn on the primary side switch S1. As shown in FIG. 4C, inthe extremely light load state, the control circuit 140 directlygenerates the first control signal Sc1 to turn on the primary sideswitch S1 after a longer blanking time starting from the reference timepoint without considering the turn on signal Vy.

Referring to the FIG. 1-5, when the secondary current Is becomes zero,the drain-source voltage Vds1 of the primary side switch S1 starts tooscillate at the same time. Therefore, in some embodiments, the statedetection circuit 130 detects the drain-source voltage Vds1 of theprimary side switch S1 and records the time point when the drain-sourcevoltage Vds1 of the primary side switch S1 starts to oscillate as thereference time point.

According to one aspects of the present application, the state detectioncircuit 130 includes a sensing capacitor Cs and a comparator 131. Thefirst terminal of the sensing capacitor Cs is electrically coupled tothe primary winding M1 and the primary side switch S1. The secondterminal of the sensing capacitor Cs is electrically coupled to thefirst terminal of the comparator 131. The first terminal of thecomparator 131 is further electrically coupled to a voltage source V1through a resistor R1, and electrically coupled to a ground terminalthrough a resistor R2. The second terminal of the comparator 131 iselectrically coupled to a reference voltage Vref1. In some embodiments,the state detection circuit 130 further includes a signal processcircuit 132. The signal process circuit 132 is connected to the outputterminal of the comparator 131. When the drain-source voltage Vds1 ofthe primary side switch S1 starts to oscillate, the sensing capacitorgenerates corresponding voltage change and current change. The currentIa will flow through the sensing capacitor Cs. When the voltage Vadecreases and is less than the reference voltage Vref1, the comparator131 outputs a trigger signal Sa to the signal process circuit 132, andthe signal process circuit 132 outputs the start signal TB_start to thecontrol circuit 140 according to the trigger signal Sa.

Referring to FIG. 6, FIG. 6 is a schematic diagram of a converter 100 insome embodiments of the present disclosure. In FIG. 6, similar elementsrelated to the embodiment of FIG. 1 are assigned with the same referencenumerals for better understanding. The specific principles of similarelements have been described in detail in the previous paragraphs, itwill not be described herein.

As shown in FIG. 6, the transformer 110 further includes a primaryauxiliary winding M3 and the state detection circuit 130 includes acomparator 131. The two terminals of the primary auxiliary winding M3respectively connect to the first input terminal of the comparator 131and the primary ground terminal, and the second terminal of thecomparator 131 is connected to the primary ground terminal. In someembodiments, the state detection circuit 130 further includes a signalprocess circuit 132. The signal process circuit 132 is connected to theoutput terminal of the comparator 131. As shown in FIG. 7, when thesecondary current Is becomes zero, the drain-source voltage Vds1 of theprimary switch S1 and the cross voltage Vaux of the primary auxiliarywinding M3 start to oscillate at the same time. When the cross voltageVaux of the primary auxiliary winding M3 starts to oscillate and crosszero voltage, the comparator 131 outputs a trigger signal Sa to thesignal process circuit 132. Then, the signal process circuit 132 outputsthe start signal TB_start to the control circuit 140 to record thereference time point. The reference time point is the time point whenthe cross voltage Vaux cross zero voltage. In some embodiments, thesecond terminal of the comparator 131 is connected to a referencevoltage Vref2, when the cross voltage Vaux of the primary auxiliarywinding M3 starts to oscillate and cross the reference voltage Vref2,the comparator 131 outputs a trigger signal Sa to the signal processcircuit 132.

In some embodiments, the state detection circuit 130 is also configuredto detect the drain-source voltage Vds2 of the secondary side switch S2and records the time point when the drain-source voltage Vds2 of thesecondary side switch S2 starts to oscillate as the reference timepoint. Referring to FIG. 8 and FIG. 9, the converter 100 includes asignal process circuit 132, and the load detection circuit 120, thestate detection circuit 130 and the peak/valley detection circuit 142are integrated in the signal process circuit 132. The signal processcircuit 132 is electrically coupled to the two terminals of thesecondary side switch S2 so as to detect the drain-source voltage Vds2of the secondary side switch S2. The load detection circuit 120, thestate detection circuit 130 and the peak/valley detection circuit 142may respectively output corresponding load state signal Vfb, startsignal TB_start and turn on signal Vy according to the drain-sourcevoltage Vds2 of the secondary side switch S2.

Further, the state detection circuit 130 is electrically coupled to thetwo terminals of the secondary side switch S2 to detect the drain-sourcevoltage Vds2 of the secondary side switch S2. As shown is FIG. 9, whenthe secondary current Is becomes zero, the drain-source voltage Vds2 ofthe secondary side switch S2 starts to oscillate correspondingly and theoscillating phase is opposite to the drain-source voltage Vds1 of theprimary side switch S1. By detecting the time when the drain-sourcevoltage Vds2 of the secondary side switch S2 starts to oscillate, thestate detection circuit 130 outputs the start signal TB_start throughthe signal processing circuit 132 to record the reference time point. Insome other embodiments, the state detection circuit 130 is electricallycoupled to the secondary side switch S2 so as to detect the time pointwhen the secondary current Is of the secondary winding becomes zero, andrecord the time point when the secondary current Is of the secondarywinding becomes zero as the reference time point.

The connection relationship and the specific structure of the loaddetection circuit 120 are not the limitations of the present disclosure.One skilled in the art can understand the configuration of the loaddetection circuit 120 and therefore will not be described here. In someembodiments, as shown in FIG. 8 and FIG. 9, the load detection circuit120 detects the negative peak value of the drain-source voltage Vds2 ofthe secondary side switch S2, and outputs the load state signal Vfbaccording to the negative peak value of the drain-source voltage Vds2 ofthe secondary side switch S2. Because the load state signal Vfb isproportional to the peak current Ipk of the primary side switch S1, andthe formula below is satisfied:

Vfb=K1×Rcs×Ipk

Isk=n×Ipk

Vds2 min=Rds×Isk

In the above three formulae, K1 is a coefficient, Rcs is sense resistorto detect the peak current of the primary side switch, n is the turnratio of transformer 110, Rds is the on-resistance value of secondaryside switch S2. Isk is the peak value of the secondary current Is. Ipkis the peak value of primary current Ip. Vds2min is the negative peakvalue of the drain-source voltage Vds2 of the secondary side switch S2.According to these formulae, the relationship between Vds2min and loadstate signal Vfb can be obtained:

$V_{{ds2}\min} = {\frac{n}{K_{1}}\frac{R_{ds}}{R_{cs}}V_{fb}}$

Accordingly, the load detection circuit 120 can output the load statesignal Vfb according to the negative peak value of the drain-sourcevoltage Vds2 of the secondary side switch S2.

As shown in FIG. 9, since the secondary side switch S2 and the primaryside switch S1 starts to oscillate at the same time, the time point whenthe drain-source voltage Vds2 of the secondary side switch S2 is at thepeak of resonance is the same as the time point when the drain-sourcevoltage Vds1 of the primary side switch S1 is at a valley. So thecontrol circuit 140 can detect whether the drain-source voltage Vds2 ofthe secondary side switch S2 is at the peak or the valley of theresonance through a peak/valley detection circuit 142. The controlcircuit 140 outputs the first control signal Sc1 to turn on the primaryside switch S1 when the drain-source voltage of the secondary sideswitch S2 is at the peak of the resonance after the blanking timestarting from the reference time point.

Referring to FIG. 10, FIG. 10 is a flowchart illustrating a controlmethod in some embodiments of the present disclosure. For ease andclarity of explanation, the following control method is described inconjunction with the embodiments shown in FIGS. 1, 6 and 8, but is notlimited thereto. Anyone who is familiar with this skill, within thespirit and scope of the present disclosure, can make various changes andretouching.

First, in step S01, the load detection circuit 120 is configured todetect the load state, and outputs the load state signal Vfbcorrespondingly. The load state signal Vfb is configured to indicate theoutput power. In some embodiments, as shown in FIG. 1, the loaddetection circuit 120 detects the voltage of the two terminals of theload. In some embodiments, as shown in FIG. 8, the load detectioncircuit 120 detects the drain-source voltage Vds2 of the secondary sideswitch S2 to calculate the load state signal Vfb.

In step S02, the control circuit 140 is configured to receive the loadstate signal Vfb, and sets the blanking time according to the load statesignal Vfb. The length of the blanking time can change with the loadstate, such as the heavy load state, the medium load state, light loadstate or extremely light load state.

In step S03, the state detection circuit 130 is configured to detect thereference time point. The reference time point is corresponding to thetime point when the secondary current Is of the secondary winding M2 ofthe transformer 110 drops to zero. In some embodiments, as shown FIG. 1,when the secondary current Is becomes zero, the drain-source voltageVds1 of the primary side switch S1 oscillates at the same time.Therefore, the state detection circuit 130 detects the drain-sourcevoltage Vds1 of the primary side switch S1, and record the time pointwhen the drain-source voltage Vds1 of the primary side switch S1 startsto oscillate as the reference time point. In some embodiments, as shownin FIG. 8, when the secondary current Is becomes zero, the drain-sourcevoltage Vds2 of the secondary side switch S2 oscillates at the sametime. Therefore, the state detection circuit 130 detects thedrain-source voltage Vds2 of the secondary side switch S2, and recordsthe time point when the drain-source voltage Vds2 of the secondary sideswitch S2 starts to oscillate as the reference time point.

In step S04, the control circuit 140 outputs the first control signalSc1 to the primary side switch S1 so as to turn on or turn off theprimary side switch S1, and the primary side switch S1 is turned on whenthe drain-source voltage Vds1 of the primary side switch S1 is at avalley after the blanking time starting from the reference time point.In some embodiments, as shown in FIG. 1, the control circuit 140 detectsthe time when the drain-source voltage Vds1 of the primary side switchS1 is at valley by the valley detection circuit 141. In someembodiments, as shown in FIG. 8 and FIG. 9, since the drain-sourcevoltage Vds2 of the secondary side switch S2 also oscillates when thesecondary current Is becomes to zero and the oscillating phase isopposite to the drain-source voltage Vds1 of the primary side switch S1,the control circuit 140 may detect the time point when the drain-sourcevoltage Vds2 of the secondary side switch S2 is at the peak of theresonance and generates the first control signal Sc1 to turn on theprimary side switch S1 after the blanking time starting from thereference time point.

FIG. 11 is a schematic diagram of a converter 100 in some otherembodiments of the present disclosure. In this embodiment, the converter100 includes a transformer 110, an active clamp circuit 160, a primaryside switch S1, a load detection circuit 120, a state detection circuit130 and a control circuit 140. Same as the above embodiments, thetransformer 110 includes a primary winding M1 and a secondary windingM2. The transformer 110 is configured to transmit the received powerfrom the primary winding M1 to the secondary winding M2. In FIG. 11, thesimilar components associated with the embodiment of FIG. 1 are labeledwith the same numerals for ease of understanding. The specific principleof the similar component has been explained in detail in the previousparagraphs, and unless it has a cooperative relationship with thecomponents of FIG. 11, it is not repeated here.

The active clamp circuit 160 is electrically coupled to the primarywinding M1, and includes a clamp capacitor Cc and a clamp switch S3. Theclamp capacitor Cc is electrically connected in series with the clampswitch S3. The clamp switch S3 is controlled by the third control signalSc3 generated by the control circuit 140. The clamp switch S3 may beMOSFET or GaN device, or MOSFET with reverse-parallel diode or GaNdevice with reverse-parallel diode. The clamp switch S3 is turned onwhen the third control signal Sc3 has a first level (e.g., high level).Relatively, the clamp switch S3 turns off when the third control signalSc3 has a second level (e.g., low level). Before the primary side switchS1 is turned on, the clamp switch S3 can be turned on, so that theenergy stored in the clamping capacitor Cc can be discharged. At thesame time, the magnetizing current Im increases negatively, so as toachieve ZVS for the primary side switch S1.

In order to decrease the switching loss, the optimal time to turn on theclamp switch S3 is when the drain-source voltage Vds1 of the primaryside switch S1 at peak value of the resonance after the blanking timestarting from the reference time point. In the embodiments, thereference time point can be detected by the state detection circuit 130same as embodiments mentioned above. For example, the reference timepoint is corresponding to a time point when the secondary current Is inthe secondary winding M2 drops to zero; or the reference time point iscorresponding to a time point when drain-source voltage Vds of theprimary side switch starts to oscillate. The load detection circuit 120is configured to detect the state of the load, and correspondinglyoutputs a load state signal Vfb, which is same as embodiments mentionedabove. The control circuit 140 confirms the present load state of theconverter 100, such as light load state, medium load state or heavy loadstate, according to the load state signal Vfb so as to set the blankingtime. Then the control circuit 140 outputs the third control signal Sc3to turn on the clamp switch S3 when the drain-source voltage Vds of theprimary side switch is at the peak value of the resonance after theblanking time starting from the reference time point.

Accordingly, since the control circuit 140 sets the blanking timeaccording to the load state, and the blanking time is not affected bythe switching frequency of the primary side switch S1. The convertercontrol method of the present disclosure is compatible with theconverter 100 with any switching frequencies design.

Referring to FIG. 12, similar elements related to the embodiment of FIG.11 are assigned with the same reference numerals for betterunderstanding. For convenience and clarity, the waveforms of the firstcontrol signal Sc1, the third control signal Sc3, the primary sidecurrent Ip, the clamp current Ic, the secondary side current Is and thedrain-source voltage Vds1 of the primary side switch S1 of the converter100 shown in FIG. 12 will be described with the embodiments shown inFIG. 11, but not limited thereto.

At the time point t0, the secondary side switch S2 is turned on. Duringthe time t0 to t1, the converter 100 is in the state of transferringenergy to the load from the transformer 110, and the secondary currentIs decreases gradually. At the time point t1, the secondary current Isdrops to zero, and the first control signal Sc1, the third controlsignal Sc3 both are keeping low level, and the secondary side switch S2is turned off. At the time point t1, the drain-source voltage Vds1 ofthe primary side switch S1 starts to oscillate. Therefore, in someembodiments, the time point t1 can be considered as the “reference timepoint” described above.

Further, the load detection circuit 120 detects the load state of theconverter 100 and correspondingly outputs the load state signal Vfb. Atthe time point t1, the control circuit 140 sets a corresponding blankingtime according the load state signal Vfb. After the blanking time, whenthe drain-source voltage Vds1 of the primary side switch S1 is at a peakvalue of the resonance (e.g., the time point t2 in FIG. 12), the controlcircuit 140 generates the third control signal Sc3 to turn on the clampswitch S3. During time point t2-t3, the clamp capacitor Cc is dischargedby the clamp current Ic, and the magnetizing current Im increasesnegatively as shown in FIG. 11.

At time t3, the clamp switch S3 is turned off, and the primary sideswitch S1 is turn on in response to the first control signal Sc1. Whenthe third control signal Sc3 switches from high level to low level, thecontrol circuit 140 generates the first control signal Sc1 to turn onthe primary side switch S1 (the first control signal Sc1 is in highlevel), so as to allow the primary current Ip flows through the primarywinding M1 and primary side switch S1. Therefore, the drain-sourcevoltage Vds1 of the primary side switch S1 is zero.

At the time point t4, the first control signal Sc1 switches from highlevel to low level. Correspondingly, the primary side switch S1 isturned off and the primary current Ip becomes zero. During the timet4-t5, secondary side switch S2 is turned on, and the secondary currentIs flows through the secondary side switch S2. As the energy stored onthe transformer 110 is transferred to the load, the secondary current Iswill gradually decrease from its maximum value to zero.

Specifically, the converter 100 sets different length of the blankingtime according to the load state signal Vfb, which is same asembodiments mentioned above in FIG. 3. Wherein, the length of theblanking time increases as the load state decreases discontinuously.That is, there is negative correlation between the blanking time and themagnitude of the load state signal. For example, the converter 100 maywork in the heavy load state, the medium load state and the light loadstate. When the converter 100 is in the heavy load state, the controlcircuit 140 sets a heavy load time as the blanking time. When theconverter 100 is in the medium load state, the control circuit 140 setsa medium load time as blanking time. The medium load time is longer thanthe heavy load time. When the converter 100 is in light load state, thecontrol circuit 140 sets a light load time as the blanking time. Thelight load time is longer than the medium load time. When the converter100 is in very light load, the control circuit 140 generates the thirdcontrol signal Sc3 to turn on the clamp switch S3 after a longerblanking time starting from the reference time point without consideringthe peak value. In some embodiment, when the converter 100 is in verylight load, there is no need to turn on the clamp switch S3, so thecontrol circuit 140 controls the clamp switch S3 to be turned off. Thecontrol circuit 140 generates the first control signal Sc1 to turn onthe primary side switch S1 after a longer blanking time starting fromthe reference time point without considering the valley or peak.

In some embodiments, the heavier load state, the shorter blanking time.In other words, the lighter load state (e.g., the load state signal Vfbbecomes smaller), the longer blanking time. In this way, the controlcircuit 140 can adjust the length of the blanking time, according to themagnitude of the load state signal Vfb. If the load detection circuit120 detects that the converter 100 is in heavy load, the control circuit140 may set the blanking time equal to zero. In other words, in heavyload, when the control circuit 140 detects that the drain-source voltageVds1 of the primary side switch starts to oscillate, the control circuit140 outputs the third control signal Sc3 (e.g., becomes high level) toturn on the clamp switch S3.

FIGS. 13-14 are waveforms of drain-source voltage Vds1 of the primaryside switch S1, blanking signal TB2/TB6, turn on signal Vy, and thethird control signal Sc3 of the converter 100. As shown in FIG. 13, in acertain/medium load state, the control circuit 140 adjusts the blankingtime according to the load state signal Vfb, and sets the blankingsignal TB2 to low level from the reference time point (for example, thetime point when the drain-source voltage Vds1 starts to oscillate). Andwhen the peak detection circuit 141 detects the peak value of theresonance, the peak detection circuit 141 outputs the turn on signal Vy.In other words, the turn on signal Vy represents the peak value of theresonance. Wherein, during the time when the blanking signal TB2 is lowlevel, the turn on signal Vy does not work. After the blanking timestarting from the reference time point, the control circuit 140 sets theblanking signal TB2 to high level, and the turn on signal Vy can beworked. At this time, when the peak detection circuit 141 detects thatthe drain-source voltage Vds1 of the primary side switch S1 is at a peakvalue of the resonance (the worked turn on signal Vy), the controlcircuit 140 outputs the third control signal Sc3 to turn on the clampswitch S3.

As shown in FIG. 14, in the light load state, the control circuit 140directly generates the third signal Sc3 to turn on the clamp switch S3after a longer blanking time starting from the reference time pointwithout considering the turn on signal Vy.

Wherein, the peak detection circuit 141 can detect the peak value of theresonance, and also can confirm the peak according to the valley of theresonance. Wherein, the peak detection circuit 141 in FIG. 1 and thevalley detection circuit in FIG. 11 can be a same circuit, which canrealize the detection of the valley and peak of the resonance. Further,the control circuit 140 can output the first control signal Sc1 based onthe turn on signal Vy reflecting information of the valley of theresonance; and/or the control circuit 140 can output the third controlsignal Sc3 based on the turn on signal Vy reflecting information of thepeak value of the resonance.

In some embodiments, in the extremely light load state, after a longerblanking time starting from the reference time point without consideringthe turn on signal Vy, the control circuit 140 directly generates thefirst signal Sc1 to turn on the primary side switch S1 withoutgenerating the third signal Sc3 to turn on the clamp switch S3. In otherwords, there may be no need to turn on the clamp switch S3 in theextremely light load state, so the control circuit 140 controls theclamp switch S3 to be turned off.

Referring to the FIG. 15, when the secondary current Is becomes zero,the drain-source voltage Vds1 of the primary side switch S1 starts tooscillate at the same time. Therefore, in some embodiments, the statedetection circuit 130 detects the drain-source voltage Vds1 of theprimary side switch S1 and records the time point when the drain-sourcevoltage Vds1 of the primary side switch S1 starts to oscillate as thereference time point. Since the method of the state detection circuit130 to detect the drain-source voltage Vds1 has been described in theforegoing embodiment, it will not be repeated here.

FIG. 16 is a schematic diagram of a converter 100 in some embodiments ofthe present disclosure. In FIG. 16, similar elements related to theembodiment of FIG. 6 are assigned with the same reference numerals forbetter understanding. The specific principles of similar elements havebeen described in detail in the previous paragraphs, it will not bedescribed herein. FIG. 17 is waveforms of the secondary side current Is,the drain-source voltage Vds1 of the primary side switch, the crossvoltage Vaux of the primary auxiliary winding, trigger signal Sa andstart signal TB_start in some embodiments of the present disclosure.

As shown in FIG. 17, when the secondary current Is becomes zero, thedrain-source voltage Vds1 of the primary switch S1 and the cross voltageVaux of the primary auxiliary winding M3 start to oscillate at the sametime. When the cross voltage Vaux of the primary auxiliary winding M3starts to oscillate and cross zero voltage, the comparator 131 outputs atrigger signal Sa to the signal process circuit 132. Then, the signalprocess circuit 132 outputs the start signal TB_start to the controlcircuit 140 to record the reference time point. The reference time pointcan be the time point when the cross voltage Vaux cross zero voltage. Insome embodiments, the second terminal of the comparator 131 is connectedto a reference voltage Vref2, when the cross voltage Vaux of the primaryauxiliary winding M3 starts to oscillate and cross the reference voltageVref2, the comparator 131 outputs a trigger signal Sa to the signalprocess circuit 132.

In some embodiments, the state detection circuit 130 is also configuredto detect the drain-source voltage Vds2 of the secondary side switch S2and records the time point when the drain-source voltage Vds2 of thesecondary side switch S2 starts to oscillate as the reference timepoint.

FIG. 18 is a schematic diagram of a converter in some embodiments of thepresent disclosure. In FIG. 18, similar elements related to theembodiment of FIG. 8 are assigned with the same reference numerals forbetter understanding. The specific principles of similar elements havebeen described in detail in previous paragraphs, it will not bedescribed herein.

Referring to FIG. 18, the load detection circuit 120, the statedetection circuit 130 and the peak/valley detection circuit 142 a mayrespectively output corresponding load state signal Vfb, start signalTB_start and turn on signal Vy according to the drain-source voltageVds2 of the secondary side switch S2.

When the secondary current Is becomes zero, the drain-source voltageVds2 of the secondary side switch S2 starts to oscillate correspondinglyand the oscillating phase is opposite to the drain-source voltage Vds1of the primary side switch S1. By detecting the time when thedrain-source voltage Vds2 of the secondary side switch S2 starts tooscillate, the state detection circuit 130 outputs the start signalTB_start through the signal processing circuit 132 to record thereference time point. In some other embodiments, the state detectioncircuit 130 is electrically coupled to the secondary side switch S2 soas to detect the time point when the secondary current Is of thesecondary winding becomes zero, and record the time point when thesecondary current Is of the secondary winding becomes zero as thereference time point.

The connection relationship and the specific structure of the loaddetection circuit 120 are not the limitations of the present disclosure.One skilled in the art can understand the configuration of the loaddetection circuit 120 and therefore will not be described here.According to the formulae mentioned above, the relationship betweennegative peak value Vds2min of the drain-source voltage Vds2 of thesecondary side switch S2 and load state signal Vfb can be obtained:

$V_{{ds2}\min} = {\frac{n}{K_{1}}\frac{R_{ds}}{R_{cs}}V_{fb}}$

In some embodiments, the load detection circuit 120 detects the negativepeak value of the drain-source voltage Vds2 of the secondary side switchS2, and outputs the load state signal Vfb according to the negative peakvalue of the drain-source voltage Vds2 of the secondary side switch S2.

Since the secondary side switch S2 and the primary side switch S1 startsto oscillate at the same time, the time point when the drain-sourcevoltage Vds2 of the secondary side switch S2 is at the valley ofresonance is the same as the time point when the drain-source voltageVds1 of the primary side switch S1 is at a peak value. Accordingly, thecontrol circuit 140 can detect whether the drain-source voltage Vds2 ofthe secondary side switch S2 is at the valley of the resonance through apeak/valley detection circuit 142 a. The control circuit 140 outputs thethird control signal Sc3 to turn on the clamp switch S3 when thedrain-source voltage of the secondary side switch S2 is at the valley ofthe resonance after the blanking time starting from the reference timepoint.

FIG. 19 is a flowchart illustrating a control method in some embodimentsof the present disclosure. For ease and clarity of explanation, thefollowing control method is described in conjunction with theembodiments shown in FIGS. 11, 16 and 18, but is not limited thereto.Anyone who is familiar with this skill, within the spirit and scope ofthe present disclosure, can make various changes and retouching.

First, in step S201, the load detection circuit 120 is configured todetect the load state, and outputs the load state signal Vfbcorrespondingly.

In step S202, the control circuit 140 is configured to receive the loadstate signal Vfb, and sets the blanking time according to the load statesignal Vfb. The length of the blanking time can change with the loadstate, such as the heavy load state, the medium load state, light loadstate or extremely light load state.

In step S203, the state detection circuit 130 is configured to detectthe reference time point. In some embodiments, the reference time pointis corresponding to the time point when the secondary current Is of thesecondary winding M2 of the transformer 110 drops to zero. In someembodiments, as shown FIG. 11, when the secondary current Is becomeszero, the drain-source voltage Vds1 of the primary side switch S1oscillates at the same time. Therefore, the state detection circuit 130detects the drain-source voltage Vds1 of the primary side switch S1, andrecord the time point when the drain-source voltage Vds1 of the primaryside switch S1 starts to oscillate as the reference time point. In someembodiments, as shown in FIG. 18, when the secondary current Is becomeszero, the drain-source voltage Vds2 of the secondary side switch S2oscillates at the same time. Therefore, the state detection circuit 130detects the drain-source voltage Vds2 of the secondary side switch S2,and records the time point when the drain-source voltage Vds2 of thesecondary side switch S2 starts to oscillate as the reference timepoint.

In step S204, when the drain-source voltage Vds1 of the primary sideswitch S1 is at a peak value after the blanking time starting from thereference time point, the control circuit 140 outputs the third controlsignal Sc3 to the clamp switch S3 so as to turn on the clamp switch S3.After a period of time, the clamp switch S3 is turned off, and theprimary side switch S1 is turn on in response to the first controlsignal Sc1.

In some embodiments, as shown in FIG. 11, the control circuit 140generates the third control signal Sc3 to turn on the clamp switch S3after the blanking time starting from the reference time point when thepeak detection circuit 141 detects the time when the drain-sourcevoltage Vds1 of the primary side switch S1 is at the peak value. In someembodiments, as shown in FIG. 18, since the drain-source voltage Vds2 ofthe secondary side switch S2 also oscillates when the secondary currentIs becomes to zero and the oscillating phase is opposite to thedrain-source voltage Vds1 of the primary side switch S1, the controlcircuit 140 may detect the time point when the drain-source voltage Vds2of the secondary side switch S2 is at the valley of the resonance andgenerates the third control signal Sc3 to turn on the clamp switch S3after the blanking time starting from the reference time point.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentdisclosure without departing from the scope or spirit of the presentdisclosure. In view of the foregoing, it is intended that the presentdisclosure cover modifications and variations of this present disclosureprovided they fall within the scope of the following claims.

What is claimed is:
 1. A converter, comprising: a transformer comprising a primary winding and a secondary winding; an active clamp circuit electrically coupled to the primary winding, and comprising a clamp switch; a primary side switch electrically coupled to the primary winding and a primary ground terminal; a secondary side switch electrically coupled to the secondary winding and a load; a load detection circuit configured to detect a load state and correspondingly output a load state signal; a state detection circuit configured to detect a reference time point; and a control circuit configured to output a control signal to turn on or turn off the clamp switch, wherein the control circuit is configured to set a blanking time according to the load state signal, such that the clamp switch is turned on when a drain-source voltage of the primary side switch is at a peak value of a resonance after the blanking time starting from the reference time point.
 2. The converter of claim 1, wherein a length of the blanking time is negative correlation with the load state.
 3. The converter of claim 2, wherein when the converter is in very light load, the control circuit controls the clamp switch to be turned off.
 4. The converter of claim 1, wherein the state detection circuit is configured to detect the drain-source voltage of the primary side switch, and records a time point when the drain-source voltage of the primary side switch starts to oscillate as the reference time point.
 5. The converter of claim 4, wherein the state detection circuit comprises a sensing capacitor and a comparator, a first terminal of the sensing capacitor is electrically coupled to the primary side switch and the primary winding, a second terminal of the sensing capacitor is electrically coupled to the comparator, when the drain-source voltage of the primary side switch starts to oscillate, the sensing capacitor generates corresponding voltage change and current change, so that the comparator correspondingly outputs a start signal to the control circuit to record the reference time point.
 6. The converter of claim 4, wherein the transformer comprises a primary auxiliary winding, the state detection circuit comprises a comparator; two terminals of the primary auxiliary winding are respectively connected to a first input terminal of the comparator and the primary ground terminal, when a cross voltage of the primary auxiliary winding starts to oscillate, the comparator correspondingly outputs a start signal to the control circuit to record the reference time point.
 7. The converter of claim 6, wherein a second terminal of the comparator is connected to the primary ground terminal, so that the comparator outputs the start signal to the control circuit to record the reference time point when the cross voltage of the primary auxiliary winding starts to oscillate and cross zero voltage.
 8. The converter of claim 6, wherein a second terminal of the comparator is connected to a reference voltage, so that the comparator outputs the start signal to the control circuit to record the reference time point when the cross voltage of the primary auxiliary winding starts to oscillate and cross the reference voltage.
 9. The converter of claim 1, wherein the state detection circuit is configured to detect a drain-source voltage of the secondary side switch and records a time point when the drain-source voltage of the secondary side switch starts to oscillate as the reference time point.
 10. The converter of claim 1, wherein the load detection circuit is configured to detect a negative peak value of a drain-source voltage of the secondary side switch, and outputs the load state signal according to the negative peak value of the drain-source voltage of the secondary side switch.
 11. The converter of claim 1, wherein the control circuit detects whether a drain-source voltage of the secondary side switch is at a valley of a resonance through a valley detection circuit, and the clamp switch is turned on when the drain-source voltage of the secondary side switch is at the valley of the resonance after the blanking time starting from the reference time point.
 12. The converter of claim 1, wherein the control circuit detects whether the drain-source voltage of the primary side switch is at the peak value the resonance through a peak detection circuit, and the clamp switch is turned on when the drain-source voltage of the primary side switch is at the peak value of the resonance after the blanking time starting from the reference time point.
 13. The converter of claim 1, wherein the state detection circuit is configured to detect a time point when a secondary current in the secondary winding drops to zero, and records the time point as the reference time point.
 14. A control method of a converter, wherein the converter comprises a primary side switch, a transformer, a secondary side switch and an active clamp circuit, the transformer comprises a primary winding and a secondary winding, the active clamp circuit is electrically coupled to the primary winding, and the active clamp circuit comprises a clamp switch, comprising: detecting a load state by a load detection circuit and correspondingly outputting a load state signal; setting a blanking time by a control circuit according to the load state signal; detecting a reference time point by a state detection circuit; and outputting a control signal to a clamp switch of the converter by the control circuit so as to turn on the clamp switch when a drain-source voltage of the primary side switch is at a peak value of a resonance after the blanking time starting from the reference time point.
 15. The control method of claim 14, further comprising: detecting the drain-source voltage of the primary side switch by the state detection circuit, and recording a time point when the drain-source voltage of the primary side switch starts to oscillate as the reference time point.
 16. The control method of claim 15, further comprising: detecting an oscillation of the drain-source voltage of the primary side switch by a sensing capacitor; and outputting a start signal to the control circuit to record the reference time point by the comparator.
 17. The control method of claim 14, further comprising: detecting an oscillation of the drain-source voltage of the primary side switch by a primary auxiliary winding of the transformer. outputting a start signal to the control circuit to record the reference time point by a comparator when detecting an oscillation of the cross voltage of a primary auxiliary winding of a transformer of the converter.
 18. The control method of claim 17, wherein the comparator outputting the start signal to the control circuit when the cross voltage of the primary auxiliary winding starts to oscillate and cross zero.
 19. The control method of claim 17, wherein the comparator outputting the start signal to the control circuit when the cross voltage of the primary auxiliary winding starts to oscillate and cross a reference voltage.
 20. The control method of claim 14, wherein a length of the blanking time is negative correlation with the load state.
 21. The control method of claim 14, further comprising: detecting a drain-source voltage of a secondary side switch of the converter by the state detection circuit; and recording a time point when the drain-source voltage of the secondary side switch starts to oscillate as the reference time point.
 22. The control method of claim 14, further comprising: detecting a time point when a secondary current in a secondary winding becomes zero; and recording the time point as the reference time point. 